Pb free solder alloy

ABSTRACT

The present invention relates to a solder alloy used for mounting or plating electronic components on a printed circuit board (PCB) and the like, and, more particularly, to a Pb-free Sn—Ag based solder alloy which comprises 0.1˜3.0 wt % of Cu, 0.01˜0.5 wt % of Ni, 0.01˜5.0 wt % of Ag, and the balance of Sn. The Pb-free Sn—Ag based solder alloy of the invention has a lower melting point, and highly enhanced wettability and joining strength, compared with the conventional Pb-free solder alloy, thereby preventing generation of bridges.

TECHNICAL FIELD

The present invention relates to a solder alloy used for mounting or plating electronic components on a printed circuit board (PCB) and the like, and, more particularly, to a Pb-free Sn—Ag based solder alloy which comprises 0.1˜3.0 wt % of Cu, 0.01˜0.5 wt % of Ni, 0.01˜5.0 wt % of Ag, and the balance of Sn.

BACKGROUND ART

As well known in the art, soldering is a joining technology using a solder alloy, in particular, for mounting miniaturized electronic components, such as semiconductor chips or resistor chips, on a printed circuit board (PCB). Recently, as electronic components are increasingly miniaturized and increased in functionality, integration of component mounting has been increased, so that a higher level of joining technology using the solder alloy is required. In other words, due to high integration of component mounting, the PCB, the electronic components mounted thereon, and the solder alloy, are more affected by cyclic stress caused by variation in temperature, thermal expansion differences, vibration, and the like, so that microstructure of the solder alloy is subjected to grain coarsening at a soldered joint, resulting in generation of cracks at the soldered joint due to fatigue. The cracks at the soldered joint act as a source of defects, such as disconnection of the electronic components mounted on the PCB.

Then, as a soldering material for mounting the electronic components on the PCB, a binary solder alloy comprising tin (Sn) and lead (Pb), such as a solder alloy comprising 60% wt Sn and 40 wt % Pb and a solder alloy comprising 63% wt Sn and 37 wt % Pb, is mainly used.

DISCLOSURE OF INVENTION Technical Problem

However, the conventional Sn—Pb based solder alloy has a problem of environmental contamination due to leakage of lead upon disposal thereof.

Moreover, as the EC directive on Waste Electrical and Electronic Equipment (WEEE) and the EC directive on the Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment (ROHS) of the European Union prohibit the use of hazardous substances in electrical and electronic equipment, it is essential that the electronic industry develop an environmentally friendly Pb-free solder alloy by restricting or excluding the use of Pb when manufacturing the solder alloy.

As for such an environmentally friendly Pb-free solder alloy, a Cu—Ni—Sn tertiary solder alloy disclosed in Japanese Unexamined Patent Publication No. 2000-225490 is well known in the art. The Pb-free Cu—Ni—Sn tertiary solder alloy is provided by substituting some contents of Cu of the conventional solder alloy with Ni, and comprises 0.05˜2.0 wt % of Cu, 0.001˜2.0 wt % of Ni, and the balance of Sn.

Since the Pb-free solder alloy does not contain Pb, it can reduce environmental contamination, and has a slightly increased mechanical strength. However, the Pb-free solder alloy disclosed therein has problems in that excessive dross is generated due to oxidation upon soldering, and in that a thread-shaped bridge connection is formed around a soldered joint due to low wettability and spreadability of the solder alloy, causing defects such as short-circuits.

Meanwhile, plating is performed on electrodes (chip components) or leads (lead elements) of the electronic components as a final process for reinforcing soldering properties and oxidation resistance (corrosion resistance). As such, when 100 wt % Sn plating or Sn—Cu plating is performed without Pb after the conventional Sn-15% Pb plating, whiskers (which are formed like whiskers on the surface of a plated portion, and grow to relieve a compressive stress when a metallic structure is subjected to compressive stress or when the surface of the plated portion is oxidized) are formed, causing short-circuits and product defects.

In order to solve the problem of the Pb-free solder alloy disclosed therein, a Pb-free quaternary solder alloy is disclosed in Korean Patent Registration No. 10-0453074, which comprises 0.05˜2.0 wt % of Cu, 0.001˜2.0 wt % of Ni, 0.001˜1.0 wt % of P, and the balance of Sn.

The Pb-free quaternary solder alloy restricts the reaction of an oxide by adding a minute amount of P to the conventional Pb-free Cu—Ni—Sn tertiary solder alloy, and increases the strength of the solder, the stress resistance of the soldered joint to endure the thermal stress and vibration around the soldered joint while reducing the fluidity of the solder alloy, thereby reducing soldering defects.

However, since the temperature is increased up to 30˜40° C. upon soldering with the Pb-free quaternary solder alloy, it is necessary for the electronic components to secure thermal resistance. Moreover, since the Pb-free quaternary solder alloy has lower wettability and spreadability (reduced about 15%) than those of the conventional Sn—Pb solder, it has deteriorated soldering properties, and if Pb-free plating (mainly, Sn plating on manual components, and Sn plating or Sn—Bi plating on IC series) is performed, the soldering properties of the components are also deteriorated.

Technical Solution

The present invention has been made in view of the above and other problems, and an aspect of the present invention is to provide a Pb-free Sn—Ag based solder alloy, which has a lower melting point, and highly enhanced wettability and joining strength, compared with the conventional Pb-free solder alloy, while having different alloy contents according to application of the solder.

DESCRIPTION OF DRAWINGS

The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a graph illustrating the relationship between wettability and Ni content;

FIG. 2 is a graph illustrating the relationship between amount of oxide and soldering time according to whether a solder alloy contains P or not; and

FIG. 3 is a graph illustrating the relationship between amount of oxide and P content.

BEST MODE

Preferred embodiment of the present invention will now be described in detail.

A Pb-free Sn—Ag based solder alloy according to the invention contains an optimum amount of Ag, Ni, and P in order to increase strength of solder alloy, and stress resistance to endure thermal stress and vibration in a soldered joint while enhancing fluidity of the solder alloy by suppressing reaction of oxide, which is a feature of the conventional Sn—Cu based Pb-free solder alloys.

Among components of the solder alloy according to the invention, since Sn has a melting point of 232° C., and is usually used as a base metal for joining, the present invention also contains Sn as the base metal. Moreover, the solder alloy of the invention comprises Cu and Ag in order to increase joining strength at the soldered joint, and Ni in order to suppress formation of Sn—Cu and Sn—Ag intermetallic compounds. Additionally, the solder alloy of the invention further comprises a minute amount of P in order to reduce generation of dross caused by friction between the surface of the solder alloy and oxygen upon soldering.

When the solder alloy comprises about 0.7 wt % of Cu and the balance of Sn, it exhibits a melting point of about 227° C., which is about 5° C. lower than the 232° C. melting point of solder alloy comprising 100 wt % of Sn.

The optimum amount of Cu added to the solder alloy is in the range of 0.3˜0.8 wt %. When 0.9 wt % or more Cu is added to the solder alloy, the melting point of the solder alloy increases again. Increased melting point of the solder alloy causes an increase of the soldering temperature, thereby having a negative influence upon electronic components having weak thermal resistance, and resulting in surface oxidation, increased viscosity, reduced wettability, and bridge or icicle-shaped defects during flow or reflow soldering. On the other hand, about 2.3 wt % Cu may be added to the solder alloy so long as thermal resistance of objects to be joined, such as PCBs, surface mounted components, metal, and the like, is secured as with a solder ball, thereby enhancing the wettability and joining strength even if soldering or metal plating is performed.

When 0.01 wt % or more Ag is added to the Sn—Cu based solder alloy, the wettability and spreadability of the solder alloy are better than those of Sn—Cu—Ni—P based solder alloy, but when the amount of Ag added to the Sn—Cu based solder alloy is less than 0.01 wt %, effect of enhancing the wettabliity and spreadability of the solder alloy is not exhibited. On the other hand, if an amount of Ag added to the Sn—Cu based solder alloy is greater than 5.0 wt %, no increase in the wettability and spreadability of the solder alloy is exhibited, and addition of large quantities of Ag is economically undesirable due to its high price. According to the invention, due to the high price of Ag, the amount of Ag added to the solder alloy is differently determined according to whether a phenol-based or epoxy-based PCB is used. An optimum amount of Ag added to the solder alloy is in the range of 0.01˜0.5 wt % (the phenol-based PCB) or in the range of 2.0˜4.0 wt % (the epoxy-based PCB).

The following Table 1 shows a wetting time and Ag costs when 0.01˜5 wt % Ag is added to a Pb-free Sn-0.5Cu-0.1Ni-0.01P solder alloy disclosed in Korean Patent Registration No. 10-0453074.

Testing was performed via soldering a base metal and a solder using a flux under the following conditions while observing joining properties between the base metal and the solder.

1) Copper plates (10×0.3 mm²) were abraded using sand paper.

2) The abraded copper plates were sufficiently dried after cleaning with I.P.A.

3) Conditions of measurement

-   -   Immersion time: 10 sec     -   Immersion depth: 2 mm     -   POT temperature: 260° C.     -   Flux property: SV-95 1F (solid form 12.5%)

TABLE 1 Average Alloy Max. Min. (sec) Price of Ag

Sn0.5Cu0.1Ni0.01P 1.61 1.27 1.42 — SnCuNiP0.01Ag 1.59 1.35 1.42 24.9 SnCuNiP0.1Ag 1.58 1.35 1.38 249 SnCuNiP0.3Ag 1.42 1.22 1.29 747 SnCuNiP0.5Ag 1.44 1.13 1.29 1,245 SnCuNiP1.0Ag 1.31 1.12 1.21 2,490 SnCuNiP2.0Ag 1.08 0.69 0.85 4,980 SnCuNiP3.0Ag 0.72 0.51 0.59 7,470 SnCuNiP3.5Ag 0.74 0.52 0.59 8,715 SnCuNiP4.0Ag 1.03 0.79 0.82 9,960 SnCuNiP5.0Ag 1.11 0.68 0.88 12,450

As shown in Table 1, the solder alloy preferably comprises 0.01˜0.5 wt % of Ag for flow soldering, and comprises 2˜4 wt % of Ag for reflow soldering.

Additionally, 0.01˜0.1 wt % of Ni is added to the solder alloy. As shown in FIG. 1, it can be seen that wettability varies according to Ni content.

Ni serves to suppress generation of intermetallic compounds, such as Sn—Cu or Sn—Ag, caused by reaction between Sn and Cu or Ag, while enhancing the fluidity of molten metal in a soldering POT.

Intermetallic compounds have a high melting point, and deteriorate the fluidity of the molten metal as well as deteriorating function of the alloy as the solder due to the presence of intermetallic compounds in the molten metal when melting the alloy. As a result, when intermetallic compounds remain between solder patterns during soldering, they form bridges, thereby causing conductors to be shorted, and when they are separated from molten solder, protrusion-shaped horns can remain.

Additionally, according to the invention, a minute amount of P is added to the solder alloy. In FIGS. 2 and 3, variation in amount of oxide according to P content is shown.

When nitrogen is used by a soldering apparatus during soldering, only the small amount of oxide is generated, thereby making it unnecessary to include P. On the other hand, if nitrogen is not used, the molten solder is oxidized during soldering due to an excessive amount of Sn in the range of 99 wt % of more, and generates the dross or the oxide, thereby forming the bridges during soldering, while deteriorating quality of the solder alloy. Accordingly, in the case where nitrogen is not used during soldering, P is added to the solder alloy, thereby minimizing generation of the dross in the soldering POT and preventing formation of the bridges. On the other hand, if P content is 0.05 wt % or more, there is no effect of preventing oxidation of the solder alloy.

The following Table 2 shows a melting point of solder alloys according to compositions of Pb-free Sn—Ag based solder alloys of the invention.

TABLE 2 No. Sn Cu Ni Ag P Melting point(° C.) 0.5% 0.05% 0.3% 1 Balance 0.3~0.8 0.01~0.1 0.01~0.5 — 227 0.5% 0.05% 0.3% 0.008% 2 Balance 0.3~0.8 0.01~0.1 0.01~0.5 0.003~0.01 227 0.5% 0.05% 3.0% 3 Balance 0.3~0.8 0.01~0.1 2.0~4.0 — 220 0.5% 0.05% 3.0% 0.008  4 Balance 0.3~0.8 0.01~0.1 2.0~4.0 0.003~0.01 220 2.3% 0.05% 0.3% 0.008% 5 Balance 2.1~2.5 0.01~0.1 0.01~0.5 0.003~0.01 270 2.3% 0.05% 3.0% 0.008% 6 Balance 2.1~2.5 0.01~0.1 2.8~3.2 0.003~0.01 270 2.3% 0.05% 0.3% 7 Balance 2.1~2.5 0.01~0.1 0.01~0.5 — 270 2.3% 0.05% 3.0% 8 Balance 2.1~2.5 0.01~0.1 2.8~3.2 — 270

Additionally, according to the invention, the Pb-free Sn—Ag based solder alloy may have different compositions according to whether the phenol-based or the epoxy-based PCB is used, whether thermal resistance of an object to be plated, such as lead of elements, flexible printed circuit (FPC), Wire-B (Wire-Bonding), Solder Ball (S/B) and the like, is secured or not, or whether nitrogen is used or not by the soldering apparatus upon soldering.

The following Table 3 shows compositions of the solder alloys according to application of the Pb-free Sn—Ag based solder alloy of the invention.

TABLE 3 Solder Alloy Application No. Sn Cu Ni Ag P Phenol Epoxy Plating S/B Wire-B 1 Balance 0.5% 0.05% 0.3% 0 X 0 0 0 2 Balance 0.5% 0.05% 0.3% 0.008% 0 X 0 0 0 3 Balance 0.5% 0.05% 3.0% 0 0 0 0 0 4 Balance 0.5% 0.05% 3.0% 0.008% 0 0 0 0 0 5 Balance 2.3% 0.05% 0.3% 0.008% 0 X 0 0 0 6 Balance 2.3% 0.05% 3.0% 0.008% 0 0 0 0 0 7 Balance 2.3% 0.05% 0.3% 0 X 0 0 0 8 Balance 2.3% 0.05% 3.0% 0 0 0 0 0

Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.

MODE FOR INVENTION

In accordance with the present invention, these and/or other aspects are accomplished by the provision of a Pb-free Sn—Ag based solder alloy, comprising: 0.1˜3.0 wt % of Cu, 0.01˜0.5 wt % of Ni, 0.01˜5.0 wt % of Ag, and the balance of Sn.

The Pb-free Sn—Ag based solder alloy may comprise 0.3˜0.8 wt % of Cu, 0.01˜0.1 wt % of Ni, 0.01˜0.5 wt % of Ag, and the balance of Sn. The Pb-free Sn—Ag based solder alloy may comprise 0.3˜0.8 wt % of Cu, 0.01˜0.1 wt % of Ni, 2.0˜4.0 wt % of Ag, and the balance of Sn.

The Pb-free Sn—Ag based solder alloy may comprise 2.1˜2.5 wt % of Cu, 0.01˜0.1 wt % of Ni, 2.0˜4.0 wt % of Ag, and the balance of Sn.

The Pb-free Sn—Ag based solder alloy may comprise 2.1˜2.5 wt % of Cu, 0.01˜0.1 wt % of Ni, 0.01˜0.5 wt % of Ag, and the balance of Sn.

The Pb-free Sn—Ag based solder alloy may further comprise 0.003˜0.01 wt % of P.

INDUSTRIAL APPLICABILITY

As apparent from the above description, the Pb-free Sn—Ag based solder alloy of the invention has a lower melting point, and highly enhanced wettability and joining strength, compared with the conventional Pb-free solder alloy, thereby preventing generation of the bridges.

Additionally, according to the invention, the Pb-free Sn—Ag based solder alloy can have the optimal composition according to whether the phenol-based or epoxy-based PCB is used, whether the thermal resistance of the object to be plated, such as leads of elements, flexible printed circuit (FPC), Wire-B (Wire-Bonding), Solder Ball (S/B) and the like, is secured or not, or whether nitrogen is used or not by the soldering apparatus upon soldering.

Accordingly, the Pb-free Sn—Ag based solder alloy of the invention can be applied to the components in various manners, such as flow soldering, reflow soldering, plating after soldering, the solder ball, the wire bonding, and the like. 

1. A Pb-free Sn—Ag based solder alloy, comprising: 0.1˜3.0 wt % of Cu; 0.01˜0.5 wt % of Ni; 0.01˜5.0 wt % of Ag; and the balance of Sn.
 2. The solder alloy according to claim 1, wherein the Pb-free Sn—Ag based solder alloy comprises 0.3˜0.8 wt % of Cu, 0.01˜0.1 wt % of Ni, 0.01˜0.5 wt % of Ag, and the balance of Sn.
 3. The solder alloy according to claim 1, wherein the Pb-free Sn—Ag based solder alloy comprises 0.3˜0.8 wt % of Cu, 0.01˜0.1 wt % of Ni, 2.0˜4.0 wt % of Ag, and the balance of Sn.
 4. The solder alloy according to claim 1, wherein the Pb-free Sn—Ag based solder alloy comprises 2.1˜2.5 wt % of Cu, 0.01˜0.1 wt % of Ni, 0.01˜0.5 wt % of Ag, and the balance of Sn.
 5. The solder alloy according to claim 1, wherein the Pb-free Sn—Ag based solder alloy comprises 2.1˜2.5 wt % of Cu, 0.01˜0.1 wt % of Ni, 2.8˜3.2 wt % of Ag, and the balance of Sn.
 6. The solder alloy according to any one of claims 1 to 5, further comprising: 0.003˜0.01 wt % of P. 